The primary aims of the proposed study are to: 2) Identify a group of women at high risk for breast cancer by detecting an intermediate marker, proliferative breast disease, in women with prior minimal cancer in one breast. 2) Perform a short-term placebo-controlled randomized chemoprevention trial with tamoxifen to evaluate its ability to modify proliferative breast disease in these women. Women who have undergone primary surgical/radiation therapy for unilateral invasive cancer of minimal extent (local disease with tumor diameter of 2 cm or less) or carcinoma in situ will be evaluated using systematic four-quadrant fine-needle aspiration cytologic sampling of the opposite breast. 120 women with moderate to severe or atypical ductal hyperplasia will be randomly assigned to receive either tamoxifen or placebo and followed with serial fine-needle aspirates to determine the effect on this intermediate endpoint. This trial has implications for the mechanism of action of anti-estrogens in chemoprevention, and if successful will support chemoprevention trials of anti-estrogens with breast cancer as an endpoint.